2. Objective
• At the end of this chapter the student is expected to:
• Define screening
• Identify diseases appropriate for screening
• Know the mechanisms of determining the validity of
screening tests
• Discuss the criteria for establishing screening program
• Describe how to evaluation screening programs
3. Definition
Screening is the search for unrecognized disease or defect by
means of rapidly applied tests, examinations or other
procedures in apparently healthy individuals.
A screening test is not intended to be diagnostic.
Screening is an initial examination only, and positive
responders require a second, diagnostic examination
4. Diseases Appropriate for Screening
To be appropriate for screening:
A disease should be serious
Treatment given before symptoms develop should be more
beneficial in terms of reducing morbidity or mortality than that
given after they develop
The prevalence of preclinical disease should be high among
the population screened
5. Criteria for establishing screening program
• The condition sought should be an important health problem.
• There should be an accepted treatment for patients with
recognized disease
• Facilities for diagnosis and treatment should be available
• There should be a recognized latent or early symptomatic
stage
• There should be a suitable test or examination which has high
validity as measured by its sensitivity and specificity
• The test should be acceptable to the population
6. • The natural history of the condition should be adequately
understood
• The cost of case-finding (including diagnosis and treatment of
patients diagnosed) should be economically balanced in
relation to possible expenditure on medical care as a whole.
• Case finding should be a continuous process and not a “once
and for all” project
7. Screening Tests
• For a screening test to be successful a suitable screening test
must be available.
• A screening test should ideally be inexpensive, easy to
administer, and impose minimal discomfort on the patients.
• In addition the results of the screening test must be valid and
reliable.
8. Screening Test
Validity of a test is the ability to differentiate accurately
between those who have the disease and those who do not.
Sensitivity and Specificity are two measures of the validity of
a screening test.
A. Sensitivity - is the ability of a test to identify correctly those
who have the disease
B. Specificity - is the ability of a test to identify correctly those
who do not have the disease
11. Predictive Value of a Screening Test
Predictive value is the ability of a test to predict the presence
or absence of disease from test results.
Predictive Value of a Positive Test (PVPT) or Positive
Predictive Value
PVPT shows the probability that the person tested positive
by this specific test truly has the disease.
PVPT = TP
TP + FP
12. Predictive Value of a Negative Test (PVNT) or Negative
Predictive Value
PVNT Shows the degree of confidence the disease can be
ruled out by using this specific test.
TN
PVNT = X 100
TN+FN
13. Reliability (Precision)
A reliable screening test is one that gives consistent results
when the test is performed more than once on the same
individual under the same conditions.
Two major factors affect consistency of results: the variation
inherent in the method and observer variation (observer
error).
1. The variability of a method- depends on such factors as the
stability of the reagents used and fluctuation in the substance
being measured ( e.g in relation to meals, diurnal variation).
2. Observer variation- can stem from differences among
observers (interobserver variation) and also from variation in
readings by the same observer on separate occasions
(intraobserver variation).
14. These variations can usually be reduced by:
1. careful standardization of procedures
2. an intensive training period for all observers (or interviewers)
3. periodic checks on their work
4. the use of two or more observers making independent
observations
15. Evaluation of screening program
Evaluation of a screening program involves consideration of
two issues: first, whether the program is feasible, and second,
whether it is effective. Both must be considered carefully
Feasibility
The feasibility of a screening program is determined by a
number of factors related to program performance, which
measure the acceptability of the program to the potential
screenees, cost-effectiveness, the subsequent diagnosis and
treatment of individuals who test positive, and the yield of
cases.
16. The acceptability of the program can be measured by factors
such as the number of persons examined and the proportion of
the target population that is screened.
The costs of the screening program must be considered in
terms of total costs as well as with regard to resources
expended per detected case of the disease.
The successful screening program must also include provision
for follow up of persons whose screening tests are positive.
This can be measured by considering the proportions of those
with positive tests who are followed, diagnosed, and treated.
17. The yield of the screening program should be high.
Yield is the number of cases detected by the screening
program.
Persons with the disease X 100
Yield = Total screened detected by the test
TP
• Yield = X 100
TP + FN + TN + FP
Effectiveness
The evaluation of the effectiveness of a screening program
must be based on measures that reflect the impact of a
program on the course of a disease.
An effective screening prograshould result in reduction of
morbidity, mortality and disability.
18. Exercise
1. Suppose a new test named “Test X" is developed for screening
of cervical cancer.
This new test was applied on 500 known early cases of
cervical cancer and 550 women with out cervical cancer.
The test was positive for 450 women with cervical cancer
and 30 women with out cervical cancer.
– Show the results by 2X2 table
– How do you determine the validity of Test X? Calculate
and interpret the measures of validity.
– How do you determine the predictive value of Test X?
Calculate and interpret the predictive value of Test X.
2. Do you recommend screening program for Syphilis? Why?
